Complete and partial spontaneous regression of metastases of lung cancer in a patient: Serial CT and F-fluodeoxyglucose PET/CT findings.

Spontaneous regression of cancer is a rare biological phenomenon and the mechanisms underlying it are poorly understood. There have been few reports of temporal changes in morphology and metabolism associated with spontaneous regression. Here, we report an 80-year-old man who presented with right upper quadrant pain. He was diagnosed with stage IVA lung cancer, but without treatment, rib metastasis disappeared 4 months after the diagnosis. Although mediastinal lymph node metastasis regressed partially it began to grow 10 months after the diagnosis. In this case, complete and partial spontaneous tumor regressions were observed in the patient, allowing for a comparison of morphological and metabolic changes during each occurrence by serial computed tomography (CT) and F-fluodeoxyglucose positron emission tomography with computed tomography (FDG-PET/CT). We observed that the rib metastasis with high FDG uptake on initial PET/CT was composed of cancer cells as well as intratumoral immune cells, whereas recurrent mediastinal lymph node metastasis with high FDG uptake on follow-up PET/CT was composed of cancer cells with few immune cells. Our findings suggest that hypermetabolism within the rib metastasis on initial PET/CT reflected immune activation, whereas hypermetabolism within the mediastinal lymph node on follow-up PET/CT reflected tumor activation.